Retroreflective photoelectric sensor

ABSTRACT

A retroreflective photoelectric sensor may be of a biaxial type or of a coaxial type. A sensor of the biaxial type includes a light-emitting optical system having a light-emitting element, a first polarizer and a light-emitting lens arranged sequentially in this order, a light-receiving optical system having a light-receiving lens, a second polarizer and a light-receiving element arranged sequentially in this order, and a phase shifter inserted between the first polarizer and the light-emitting lens of the light-emitting optical system. The polarizer axes of the first and second polarizers are mutually perpendicular. A phase shifter such as a ½ wave plate may further be placed between the first polarizer and the light-emitting lens of the light-emitting optical system.

BACKGROUND OF THE INVENTION

This invention relates to a retroreflective photoelectric sensor whichmay be used with a reflector to together sandwich a target detectionarea for detecting a light-reflecting target object in the detectionarea.

A retroreflective photoelectric sensor is used generally with areflector to together sandwich a target detection area therebetween anddetects the presence or absence of a target object of detection based onthe difference in the character of reflected light from the reflectorand that from the target object of detection for light emitted from thesensor. In the case of a retroreflective photoelectric sensor for atarget object, a reflector with a reflection characteristic that affectsthe polarization mode of the light emitted from the sensor is used, andthe presence or absence of a target object of detection is determined onthe basis of whether or not a polarization component peculiar to thereflected light from the reflector is contained in the light receivedfrom the target area of detection. For this purpose, it has been knownto use reflectors of the type, for example, having many triangularpyramid-shaped indentations distributed on the reflective surface suchthat the incident light is reflected several times on the three surfacesaround the top point of the pyramid shape to convert the polarizationmode of the incident light which is initially linearly polarized and toreturn the reflected light back in the direction from which the incidentlight came.

The present inventors have earlier proposed a retroreflectivephotoelectric sensor of a so-called biaxial kind. For this sensor, alight-emitting lens and a light-receiving lens with small polarizationdistortion with retardation value less than 17 nm, made byinjection-molding of a resin material, were used, and an optical systemfor the light emission was made by arranging a light-emitting element, afirst polarizer (say, for vertical polarization) and the light-emittinglens in this order, another optical system for the light reception beingmade by arranging the light-receiving lens, a second polarizer (say, forhorizontal polarization) and a light-receiving element, in this order.

Similarly, the present inventors also proposed (in Japanese PatentPublication Tokkai 2001-228260) a different retroreflectivephotoelectric sensor of a so-called coaxial kind, comprising alight-emitting optical system for emitting light from a light-emittingelement by passing it through a first polarizer (say, for verticalpolarization), a light-receiving optical system for receiving lightthrough a second polarizer (say, for horizontal polarization) andconverting the received light into an electrical signal by using alight-receiving element, a single common lens for both emitting lightfrom the light-emitting element and receiving light to be received bythe light-receiving element and a beam splitter placed between thelight-emitting and light-receiving optical systems and the common lensfor directing both the outgoing light from the light-emitting element tothe common lens and the incoming light through the common lens to thelight-receiving optical system.

A sensor with this structure is advantageous in that the first andsecond polarizers, which used to be placed in front of the respectivelenses according to the earlier technology, are now placed behind thelens and they may be made smaller in size and hence that the sensor caseand the lens can be integrally formed, thereby contributing tosignificantly reduce the production cost of the sensor.

With a sensor of this structure with both the first and secondpolarizers placed behind the lens, however, there was a problem of lightleakage even if the two polarizers are set in the mutually perpendicularrelationship (or the cross-nicol relationship, forming so-called crossednicols) such that the quantity of received light in the presence of alight-reflecting target object could not be made sufficiently smallerthan that in its absence.

SUMMARY OF THE INVENTION

It is therefore an object of this invention in view of the problemdescribed above to provide a retroreflective photoelectric sensor whichcan be produced inexpensively and still has a reliable detectioncapability.

A retroreflective photoelectric sensor embodying this invention may beof a biaxial type or of a coaxial type. A retroreflective photoelectricsensor of the biaxial type embodying this invention may be characterizedas comprising a light-emitting optical system having a light-emittingelement that emits an expanding beam of light with a fixed gate angle, afirst polarizer and a light-emitting lens arranged sequentially in thisorder, a light-receiving optical system having a light-receiving lens, asecond polarizer and a light-receiving element arranged sequentially inthis order, and a phase shifter inserted between the first polarizer andthe light-emitting lens of the light-emitting optical system, andwherein the first polarizer and the second polarizer have mutuallyperpendicular polarizer axes. Thus, light from the light-emittingelement passes through the first polarizer while increasing thesectional area of its flux to be made incidence to the light-emittinglens and is transmitted to the target area of detection. Similarly,reflected light received through the light-receiving lens passes throughthe second polarizer while decreasing the sectional area of its flux tobe made incidence to the light-receiving element serving to generate anelectrical signal according to the quantity of the received light. Thefirst and second polarizers are arranged such that their polarizer axesare perpendicular to each other, or they may be preferably set in theso-called cross-nicol relationship. A phase shifter may further beplaced between the first polarizer and the light-emitting lens of thelight-emitting optical system. A ½ wave plate (or a ½ phase shifter) maypreferably be used for this purpose.

With a sensor thus structured, rotations of the polarization plane ofthe light as it passes through the first polarizer and thelight-emitting lens, which take place mainly in the beams propagating indirections making about 45° with the polarizer axis are canceled out bymeans of the inserted phase shifter. As a result, the difference in thequantity of received light when there is and there is not a targetobject in the target area of detection becomes greater and hence thereliability of detection can be improved. Such a sensor may be describedas comprising a light-emitting optical system having a light-emittingelement, a first polarizer and a light-emitting lens arrangedsequentially, a light-receiving optical system having a light-receivinglens, a second polarizer (having a polarizer axis perpendicular to thatof the first polarizer) and a light-receiving element arrangedsequentially, and means disposed between the first polarizer and thelight-emitting lens for canceling the total rotation of the polarizationplane that is the sum of rotations caused by passing through the firstpolarizer and the light-emitting lens on the beams of light propagatingaround directions of 1 o'clock and 30 minutes, 4 o'clock and 30 minutes,7 o'clock and 30 minutes and 10 o'clock and 30 minutes (or directionsmaking angles of about 45° with the polarizer axis), depending on itsgate angle.

Another phase shifter may also be provided between the light-receivinglens and the second polarizer of the light-receiving optical system.With a sensor thus structured, rotations of the polarization plane ofthe reflected light as it passes through the light-receiving lens andthe second polarizer, which take place mainly in the beams propagatingin directions making about 45° with the polarizer axis are also canceledout. Thus, the difference in the quantity of received light when thereis and there is not a target object in the target area of detectionbecomes even greater and hence the reliability of detection can befurther improved.

If the light-emitting and light-receiving lenses are integrally formedby molding a plastic material, the polarizers may be made smaller andthe production cost may be reduced.

A retroreflective photoelectric sensor of the coaxial type embodyingthis invention may be characterized as comprising a light-emittingoptical system having a light-emitting element and a first polarizer andserving to transmit light from the light-emitting element through thefirst polarizer, a light-receiving optical system having a secondpolarizer having a polarizer axis perpendicular to that of the firstpolarizer and a light-receiving element and serving to convert lightreceived through the second polarizer into an electrical signal by thelight-receiving element, a single lens for both emitting light from thelight-emitting element and receiving light to be received by thelight-receiving element therethrough, a beam splitter serving to directlight received from the light-emitting optical system to the single lensand light received from the single lens to the light-receiving opticalsystem, and a phase shifter inserted between the first polarizer and thebeam splitter. According to this embodiment, too, light from thelight-emitting element passes through the first polarizer whileincreasing the sectional area of its flux to be made incidence to thelight-emitting lens and is transmitted to the target area of detection,and reflected light received through the light-receiving lens passesthrough the second polarizer while decreasing the sectional area of itsflux to be made incidence to the light-receiving element serving togenerate an electrical signal according to the quantity of the receivedlight. The first and second polarizers are arranged such that theirpolarizer axes are perpendicular to each other, or they may bepreferably set in the so-called cross-nicol relationship. A phaseshifter may further be placed between the first polarizer and thelight-emitting lens of the light-emitting optical system. A ½ wave plate(or a ½ phase shifter) may preferably be used for this purpose. Meritsof the sensor of the biaxial type described above are also present withthe sensor of the coaxial type.

The aforementioned phase shifter may be placed alternatively between thebeam splitter and the single lens. This embodiment is advantageouswherein the number of constituent elements is be reduced and hence theproduction cost is lowered.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic drawings for explaining the principle of aretroreflective photoelectric sensor of a biaxial type respectively whenthere is not and there is an object in the target area of detection.

FIG. 2 is a structural diagram of the optical system of aretroreflective photoelectric sensor of a biaxial type embodying thisinvention.

FIGS. 3A and 3B are schematic drawings for explaining the principle of aretroreflective photoelectric sensor of a coaxial type respectively whenthere is not and there is an object in the target area of detection.

FIG. 4 is a structural diagram of the optical system of aretroreflective photoelectric sensor of a coaxial type embodying thisinvention.

FIG. 5, consisting of FIGS. 5A, 5B and 5C, includes drawings forexplaining the change in the direction of polarization of light by apolarizer.

FIG. 6, consisting of FIGS. 6A, 6B and 6C, includes drawings forexplaining the change in the direction of polarization of light by alens.

FIG. 7, consisting of FIGS. 7A, 7B and 7C, includes drawings forexplaining the change in the direction of polarization of light by a ½phase shifter.

FIG. 8, consisting of FIGS. 8A, 8B, 8C and FIG. 9, consisting of FIGS.9A and 9B, are drawings for explaining the change in the direction ofpolarization of light by a ½ phase shifter obtained by eliminating theeffects of the lens.

FIG. 10, consisting of FIGS. 10A, 10B, 10C and FIG. 11, consisting ofFIGS. 11A and 11B, are drawings for explaining the total change in thedirection of polarization of light by a polarizer, a ½ phase shifter anda lens.

FIGS. 12A and 12B, together referred to as FIG. 12, are sectional viewsof the case of a retroreflective photoelectric sensor of the biaxialtype embodying this invention.

FIG. 13 is a sectional view of the case of a retroreflectivephotoelectric sensor of the coaxial type embodying this invention.

FIG. 14 is a structural diagram of the optical system of anotherretroreflective photoelectric sensor of a coaxial type embodying thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described next by way of an example with reference todrawings but this example is merely one of many that embody theinvention and hence is not intended to limit the scope of the invention.

FIGS. 1A and 1B are schematic drawings for explaining the principle of aretroreflective photoelectric sensor 100 of a biaxial type respectivelywhen there is not and there is an object in the target area ofdetection. In FIGS. 1A and 1B, symbol 200 indicates a reflector having areflecting surface 200 a, symbol 300 indicates a light-reflecting targetobject of detection with a reflecting surface 300 a, symbol L1 indicateslight emitted from the sensor 100, symbol L2 indicates reflected lightfrom the reflector 200 and symbol L3 indicates reflected light from thetarget object 300.

As can be clearly understood from FIGS. 1A and 1B, the retroreflectivesensor 100 is placed opposite the reflector 200 so as to sandwichtherewith a target area of detection where the target object 300 isexpected to pass. As explained above, the reflecting surface 200 a ofthe reflector 200 is formed so as not only to reflect the emitted lightL1 from the sensor 100 but also to change its polarization mode uponreflection. If the emitted light L1 from the sensor 100 is linearlypolarized in the horizontal direction, for example, the reflected lightL2 may be elliptically polarized including a vertical component.

If there is no object in the target area of detection, as shown in FIG.1A, the emitted light L1 from the sensor 100 is reflected by thereflecting surface 200 a of the reflector 200 and hence a sufficientlylarge quantity of reflected light L2 is received by the sensor 100. Ifthe emitted light L1 from the sensor 100 is linearly polarized in thevertical direction, for example, the reflected light L2 is ellipticallypolarized including a horizontal component. Thus, the sensor 100 candetermine the presence or absence of an object in the target area ofdetection on the basis of the determination whether or not a specifiedamount of horizontal component is included in the light received therebyfrom the target area of detection.

If there is a light-reflecting target object 300 in the target area ofdetection, as shown in FIG. 1B, the emitted light L1 from the sensor 100is reflected by its reflecting surface 300 a and hence a fairly largequantity of reflected light L3 is received by the sensor 100. Since thereflecting surface 300 a of the target object 300 is not specificallystructured to modify the polarization mode of the light L1 from thesensor 100 made incident thereupon, if the emitted light L1 from thesensor 100 is linearly polarized in the vertical direction, for example,the reflected light L3 from the target object 300 will also be linearlypolarized in the vertical direction, having no horizontal component.Thus, the presence of an object in the target area of detection can bedetected on the basis of the absence of horizontal component in thelight received from the target area of detection.

As shown schematically in FIG. 2, the retroreflective photoelectricsensor 100′ of a biaxial type embodying this invention comprises anoptical system for emitting light (the light-emitting optical system 1)and another optical system for receiving light (the light-receivingoptical system 2). The light-emitting optical system 1 has alight-emitting element 11, a first polarizer 12 and a light-emittinglens 14 arranged in this order, and the light-receiving optical system 2has a light-receiving lens 21, a second polarizer 23 and alight-receiving element 24 in this order. The first and secondpolarizers 12 and 23 have their polarizer axes differently oriented. Inthe present invention, the polarization axis of the first polarizer 12is vertical and that of the second polarizer 23 is horizontal. In otherwords, the first and second polarizers 12 and 23 in this example are inthe so-called cross-nicol relationship.

In addition, a half-wave phase shifter (hereinafter referred to as the“½ phase shifter”) 13 is disposed between the first polarizer 12 and thelight-emitting lens 14 of the light-emitting optical system 1 andanother ½ phase shifter 22 is disposed between the light-receiving lens21 and the second polarizer 23 of the light-receiving optical system 2.Although FIG. 2 shows the first and second polarizers 12 and 23 indirect contact respectively with the associated one of the ½ phaseshifters 13 and 22, this is not intended to limit the scope of theinvention. They may be arranged so as to be mutually separated withappropriate intervals therebetween. It is preferable to use a materialwith a small index of refraction for the light-emitting andlight-receiving lenses 14 and 21. Plastic lenses with little doublerefraction and glass lenses may be utilized.

The emitted light from the light-emitting element 11 passes through thefirst polarizer 12 and the ½ phase shifter 13 while enlarging thesectional area of its flux at a fixed rate and is made incidence on thelight-emitting lens 14, propagating thereafter as the emitted light L1to the target area of detection. Reflected light L2 or L3 from thetarget area of detection is passed through the light-receiving lens 21and thereafter through the ½ phase shifter 22 and the second polarizer23 while reducing the sectional area of its flux at a fixed rate, beingreceived by the light receiving element 24 and converted thereby into anelectrical signal. Since the first and second polarizers 12 and 23 arein the cross-nicol relationship, the presence or absence of an object inthe target area of detection can be determined as explained above withreference to FIG. 1.

Since the ½ phase shifters 13 and 22 are inserted according to thisinvention respectively between the first polarizer 12 and thelight-emitting lens 14 and between the light-receiving lens 21 and thesecond polarizer 23, leakage of light can be minimized when there is alight-reflecting target object in the target area of detection such thatthere will be a sufficiently large difference in the quantity ofreceived light between when there is and is not a target object in thetarget area of detection. This will be explained more in detail belowwith reference to FIGS. 5–11.

The present invention includes retroreflective photoelectric sensors ofthe so-called coaxial type. FIGS. 3A and 3B are schematic drawings forexplaining the principle of a retroreflective photoelectric sensor 400of the coaxial type respectively when there is not and there is anobject in the target area of detection. In FIGS. 3A and 3B, as in FIGS.1A and 1B explained above, symbol 200 indicates a reflector having areflecting surface 200 a, symbol 300 indicates a target object with areflecting surface 300 a, symbol L1 indicates light emitted from thesensor 100, symbol L2 indicates reflected light from the reflector 200and symbol L3 indicates reflected light from the target object 300.

The principles of operation are essentially the same as for a sensor ofthe biaxial type. If there is no object in the target area of detectionas shown in FIG. 3A, the sensor 400 receives reflected light L2 from thereflector 200 but if there is a target object 300 in the target area ofdetection as shown in FIG. 3B, the sensor 400 receives reflected lightL3 from the target object 300. The difference is that the reflectedlight L2 and L3 and the emitted light L1 have the same optical axis.

As shown schematically in FIG. 4, the retroreflective photoelectricsensor 400 of the coaxial type embodying this invention comprises anoptical system for emitting light (the light-emitting optical system 3)and another optical system for receiving light (the light-receivingoptical system 4). The light-emitting optical system 3 serves to outputthe light from a light-emitting element 31 through a first polarizer 32,and the light-receiving optical system 4 serves to convert receivedlight polarized by a second polarizer 42 into an electrical signal by alight-receiving element 43. The sensor 400 further comprises a singlelens (the “common lens 5”) used for passing both emitted and receivedlight and a beam splitter 6 disposed between the common lens 5 and thelight-emitting and light-receiving optical systems 3 and 4 for directingthe outgoing light emitted from the light-emitting optical system 3 tothe common lens 5 and the incoming light received through the commonlens 5 to the light-receiving optical system 4.

The polarization directions (or the directions of the polarizer axes) ofthe first and second polarizers 32 and 42 respectively of thelight-emitting and light-receiving optical systems 3 and 4 aredifferent. In the present example being described, the polarizer axis ofthe first polarizer 32 is perpendicular to the plane defined by thelight-emitting element and the common lens 5, and the polarizer axis ofthe second polarizer is horizontal.

In addition to the above, a ½ phase shifter 33 is disposed between thefirst polarizer 32 of the light-emitting optical system 3 and the beamsplitter 6 and another ½ phase shifter 41 is disposed between the secondpolarizer 42 of the light-receiving system 4 and the beam splitter 6. Itis preferable to use a material with a small index of refraction alsofor the common lens 5. A plastic lens with little double refraction or aglass lenses may be utilized.

The emitted light from the light-emitting element 31 passes through thefirst polarizer 32 and the ½ phase shifter 33 while enlarging thesectional area of its flux at a fixed rate and is made incidence to thecommon lens 5, propagating thereafter as the emitted light L1 to thetarget area of detection. Reflected light L2 or L3 from the target areaof detection is passed through the common lens 5 and thereaftersequentially through the ½ phase shifter 41 and the second polarizer 42while reducing the sectional area of its flux at a fixed rate, beingreceived by the light receiving element 43 and converted thereby into anelectrical signal according to the quantity of the received light. Sincethe first and second polarizers 32 and 42 are in the cross-nicolrelationship, the presence or absence of an object in the target area ofdetection can be determined as explained above with reference to FIG. 3.

Since the ½ phase shifters 33 and 41 are inserted according to thisinvention respectively between the first polarizer 32 and the beamsplitter 6 and between the beam splitter 6 and the second polarizer 42,the difference in the quantity of received light can be madesignificantly large between when there is and is not a light-reflectingtarget object in the target area of detection. This, too, will beexplained more in detail below with reference to FIGS. 5–11.

FIGS. 5A, 5B and 5C explain the local rotation of the plane ofpolarization (or the direction of polarization) by a polarizer. Thepresent inventors set a light-emitting element LT and a stationarycamera SC opposite each other and a first polarizer P11 and a secondpolarizer P12 in a cross-nicol relationship on the optical path betweenthe light-emitting element LT and the stationary camera SC, as shown inFIG. 5A. The image thus taken by the camera SC clearly showed thepresence of light leakage at four places corresponding to the fourcorners of a square. It is believed because the light from thelight-emitting element LT propagates with its sectional area increasingand hence does not pass through the polarizers P11 and P12perpendicularly thereto if its “gate angle” θ1 (defined, as shown inFIG. 5A, as the angle between the optical axis of the beam and the beamunder consideration off the optical axis) is large.

Let us assume that the direction of polarization (or the polarizer axis)of the first polarizer P1 is vertical and that of the second polarizerP2 is horizontal. FIG. 5B shows the directions of polarization ofdifferent portions of the emitted light from the light-emitting elementLT, the center of the circle indicating the beam of light headingperpendicularly to the first polarizer P11. The beams heading upward, tothe right, downward and to the left are respectively represented atpositions of 12 o'clock, 3 o'clock, 6 o'clock and 9 o'clock. As shown inFIG. 5B, the vertical direction of polarization of the emitted beam fromthe light-emitting element LT does not change for beams propagationstraight as well as those propagating in the directions corresponding to12 o'clock, 3 o'clock, 6 o'clock and 9 o'clock. For beams in thedirections in between (each making an angle of 45° with the polarizeraxis) corresponding to 1 o'clock and 30 minutes, 4 o'clock and 30minutes, 7 o'clock and 30 minutes and 10 o'clock and 30 minutes,however, there is a change in the direction of polarization. This changeincreases as the gate angle increases, as shown in FIG. 5C, causing thelight leakage as explained above.

The graph of FIG. 5C shows the relationship between the gate angle andthe angle of polarization at the position of 1 o'clock 30 minutes(Position A indicated in FIG. 5B). This graph shows that the directionof polarization hardly changes if the gate angle is less than 10° butchanges rapidly if the gate angle exceeds 10° and that the change in thepolarization direction is about 2° when the gate angle is 30°.

Thus, if two polarizers P11 and P12 are placed in front of thelight-emitting element LT and light therefrom is caused to pass throughthese polarizers P11 and P12, the beams separated from the optical axisof the emitted light pass through the polarizers obliquely and thepolarization direction changes significantly for such beams. This is howleakage of light results.

FIGS. 6A, 6B and 6C explain the local rotation of the plane ofpolarization by a lens. Let us consider a situation where a linearlypolarized beam of light is passed through a plano-convex lens LS (havinga flat surface S1 and a convex surface S2) from the side of its convexsurface S2, as shown in FIG. 6A. If the direction of an incident lightbeam is indicated by symbol 51 and the normal line at the point ofincidence is indicated by symbol 52, the angle of incidence θ2 for thisbeam is the angle between these two directions 51 and 52. This showsclearly that the angle of incidence θ2 increases as the point ofincidence of the beam approaches the periphery of the lens LS.

In FIG. 6A, symbol B indicates a linearly polarized beam makingincidence obliquely at an angle of 45°. The graph of FIG. 6C indicatesthat the polarization direction of such a beam changes, as explainedmore in detail, for example, in “Applied Physical Engineering” byTsuruta (published by Baiyo-kan (Fifth Edition (1998) at pages 237–240).It is explained, in particular, that: “If linearly polarized lightoscillating in the direction of 45° from the plane of incidence, thereis no retardation in phase between transmissivity tp and ts ofp-polarization and s-polarization and since tp>ts, the transmitted beamis also linearly polarized and its plane of oscillation approachesslightly the plane of incidence (θ<45°).”

FIGS. 7A, 7B and 7C explain the effect of change in the direction ofpolarization by a ½ phase shifter. Let us consider a situation where a ½phase shifter P2 is superposed on the output side of the first polarizerP11, as shown in FIG. 7A, to form a combination and light from thelight-emitting element LT is passed therethrough from the side of thefirst polarizer to the ½ phase shifter P2. Then, as explained above withreference to FIG. 5B, light being outputted from the polarizer P11 hasthe direction of its linear polarization changed at positions in thedirections of 1 o'clock and 30 minutes, 4 o'clock and 30 minutes, 7o'clock and 30 minutes and 10 o'clock and 30 minutes. On the output sideof the ½ phase shifter P2, however, there are changes in the oppositedirection. In this situation, the change in the polarization directionby the oblique transmission of the polarizer P11 and that by thetransmission of the ½ phase shifter P2 are related so as to reverse thepolarity, as shown in FIG. 7C. For example, if the change in thepolarization direction by passing through the polarizer P11 is +Δθ, thechange after the passage through the ½ phase shifter P2 is −Δθ. Thischange is illustrated in FIG. 7B for other beams.

Next, FIGS. 8A, 8B, 8C, 9A and 9B are referenced to explain theexperiment carried out by the present inventors regarding the change inthe polarization direction by a ½ phase shifter, obtained by eliminatingthe effects of the lens. In this experiment, the light-emitting elementLT and the stationary camera SC were set opposite each other and thefirst polarizer P11 and the second polarizer P12 were set in across-nicol relationship on the optical path between the light-emittingelement LT and the stationary camera SC, as shown in FIG. 5A. The ½phase shifter P2 was further placed on the output side of the firstpolarizer P11, as shown in FIG. 7A and altogether as shown in FIG. 8A.The directions of polarization observed at the output side of thepolarizer P11 and the output side of the ½ phase shifter P2 are shown inFIG. 8B.

FIG. 8B shows that changes in the polarization direction are observed atpositions in the directions of 1 o'clock and 30 minutes, 4 o'clock and30 minutes, 7 o'clock and 30 minutes and 10 o'clock and 30 minutesaccording to the gate angle. As shown in FIG. 8C, if the change in thepolarization direction due to the polarizer P11 is +Δθ, the change afterthe passage through the ½ phase shifter P2 is −Δθ. As a result, theimage taken by the camera SC by the light after passing through thesecond polarizer P12 includes four areas A11, A12, A13 and A14 withlight leakage, as shown in FIG. 9A. Symbol A2 indicates a screened area.

FIG. 9B is a graph for showing that the change in the polarizationdirection due to oblique incidence onto the polarizer is reversed inpolarity by the ½ phase shifter. In summary, as shown by FIGS. 7A and8A, the effect of the polarizer P11 on the polarization direction can bereversed by the ½ phase shifter P2 placed on the output side of thepolarizer P11.

Next, FIGS. 10A, 10B, 10C, 11A and 11B are referenced to explain thecombined effects of polarizers, a ½ phase shifter and a lens. As shownin FIG. 10A, the light-emitting element LT, the first polarizer P11, the½ phase shifter P2, the lens LS, the second polarizer P12 and thestationary camera SC were arranged in this order and light emitted fromthe light-emitting element LT was passed sequentially through the firstpolarizer P11, the ½ phase shifter P2 and the lens LS to be madeincidence on the second polarizer P12, and the output light from thesecond polarizer P12 was observed by the camera SC. As a result, asshown in FIGS. 10B and 10C, the polarization direction changes as thelight passes through the polarizer P11 at the positions in thedirections of 1 o'clock and 30 minutes, 4 o'clock and 30 minutes, 7o'clock and 30 minutes and 10 o'clock and 30 minutes according to thegate angle. After the light passes through the ½ phase shifter P2, aninversion in polarity takes place on the change in the direction ofpolarization brought about by the oblique incidence onto the polarizerP11. As the light further passes through the lens LS, the polarizationdirection changes further, depending on the angle of incidence. Sincethe change in the polarization direction by the lens LS cancels out thechange in the polarization direction brought about by the obliqueincidence onto the ½ phase shifter P2, as shown in FIG. 10C, the imageof the second polarizer P12 taken by the camera SC may be made entirelyinto the screened area A2 such that the leakage of light can becompletely prevented.

This means, as shown in FIG. 11B, that the change in the polarizationdirection can be made approximately constant, regardless of the increasein the gate angle. In other words, the change in the polarizationdirection shown in FIG. 5C brought about by the oblique incidence ontothe polarizer can be inverted by means of the ½ phase shifter as shownin FIG. 9B and further cancelled by the change depending upon the angleof incidence to the lens as shown in FIG. 9C such that the nearly flatchange characteristic as shown in FIG. 11B can be obtained finally. Inthis manner, the cross-nicol relationship between the first and secondpolarizers can be maintained nearly all over the area.

Although FIGS. 5–11 were referenced above to explain a situation wherelight passes through a polarizer, a ½ phase shifter and a lens, in thisorder, a similar result is obtained also where light passes through alens, a ½ phase shifter and a polarizer, in this order. Thus, leakage oflight can be reduced in a similar way also in the light-receivingoptical system.

In summary, a retroreflective photoelectric sensor of this inventionadopts an optical structure as characterized by FIG. 2 or 4 such thatthe leakage of light can be reduced as shown by FIG. 11 and hence thatthe presence and absence of a target object of detection can be clearlydistinguished.

As explained above with reference to FIGS. 5–11 above, the basicprinciple of the invention is to appropriately balance the change in thepolarization direction by the passage of light through a polarizer, theinversion of the polarization direction by the passage through a phaseshifter and the change in the polarization direction by the passagethrough a lens such that they will cancel out. Thus, the phase shift tobe effected by the phase shifter may be considered to be a matter ofdesign. According to the studies by the present inventors, it ispreferable that the phase shift to be effected by the phase shifter bein the range of ⅜–⅝ (with respect to the wavelength, that is, in unitsof 2π) and more particularly preferable to be closer to ½. The optimumphase shift is believed to be obtainable from the relationship betweenthe angles of incidence to the polarizer and the lens. According to thestudies by the present inventors, the angle between the polarizationaxis of the polarizer and the optical axis of the phase shifter shouldpreferably be less than about 5°.

FIG. 2 is referenced again to explain the invention more in detail.

The beam of light from the light-emitting element 11 passes through thefirst polarizer 12 and the ½ phase shifter 13 and then continues topropagate while increasing its sectional area such that it will be aboutthe same as the effective surface area of the light-emitting lens 14.The center portion of the beam (along its optical axis) makes incidenceto all of the first polarizer 12, the ½ phase shifter 13 and thelight-emitting lens 14 and hence the direction of its (linear)polarization does not change. Since the optical axis of the ½ phaseshifter is set parallel (or perpendicular) to the direction ofpolarization (polarizer axis) of the first polarizer 12, the center beamis under the same condition as if the ½ phase shifter were not presentand the beam propagates to the light-emitting lens 14 with the directionof polarization unchanged. Since this center beam makes incidence ontothe light-emitting lens 14 also perpendicularly, the beam passes throughthe center of the light-emitting lens 14 without changing the directionof its polarization.

Next, peripheral portions of the beam of light emitted from thelight-emitting element 11 (which will pass peripheral points of theeffective area of the light-emitting lens 14) will be considered. Theseportions of light do not make incidence perpendicularly onto the firstpolarizer 12 and the direction of polarization changes, depending of theangular position of the beam, as explained above with reference to FIG.5B. Consider, for example, the portion propagating in the direction of 1o'clock and 30 minutes with reference to FIG. 5B. The direction ofpolarization of this beam will not change as long as its gate angle issufficiently small (say, less than 10°) but begins to change as the gateangle increases. When the gate angle is 30°, the direction ofpolarization changes by +1.8° C.

If the ½ phase shifter 13 were not present and the aforementionedperipheral portion of the beam were passed directly through thelight-emitting lens 14, the direction of its polarization would furtherchange in the positive direction, as explained above with reference toFIG. 6. If the light-emitting lens 14 is made of an acryl resinmaterial, the change in the polarization direction will be as large as+4.0° if the angle of incidence is 68° (corresponding to the gate angleof 30°). The total change including the change by the first polarizer 12would be 1.8°+4.0°=5.8°.

If the ½ phase shifter 13 is inserted between the first polarizer 12 andthe light-emitting lens 14 according to the present invention, itfunctions to change the polarization direction in the opposite directionby the same angle by which the polarization direction was changed as thebeam passed through the first polarizer 12, as explained above withreference to FIGS. 7 and 8. If the gate angle is 30°, the polarizationdirection should change from +1.8° to −1.8°. Experimentally, however, itwas not −1.8° but −3.2°. It was probably because the ½ phase shifter waspasted onto the first polarizer 12 and there was an error in thearrangement of its optical axis.

Thereafter, the beam is passed through the light-emitting lens 14 andthe polarization direction is changed again in the positive directionand the earlier obtained change in the negative direction is cancelled.If the angle of incidence onto the light-emitting lens 14 is 68°, thepolarization direction changes by +4.0° and the total change becomes−3.3°+4.0°=+0.8°. This is much smaller than the total change of +5.8° inthe polarization direction if the ½ phase shifter 13 were not inserted.Even if the polarization direction were −1.8° after the ½ phase shifter13 is passed, the total change after the light passes through thelight-transmitting lens 114 becomes −1.8°+4.0°=+2.2° and it is stillmuch smaller than if the ½ phase shifter 13 were not inserted.

Next, FIG. 4 is referenced again to explain more in detail therelationship between the directions of the polarizer axes of the firstand second polarizers 32 and 42.

Let us assume firstly that the direction of the polarizer axis of thefirst polarizer 32 is perpendicular (in the “perpendicular direction”)to the plane defined by the light-emitting element 31, thelight-receiving element 43 and the common lens 5 and that of the secondpolarizer 32 is horizontal. In this case, the light emitted from thelight-emitting element 31 and passed through the first polarizer 32 islinearly polarized in the perpendicular direction except that thedirection of polarization changes for beams propagating in thedirections of 1 o'clock and 30 minutes, 4 o'clock and 30 minutes, 7o'clock and 30 minutes and 10 o'clock and 30 minutes (or directionsmaking 45° with the polarizer axis) if the gate angle becomessufficiently large, as explained above. After this linearly polarizedlight passes through the ½ phase shifter 33, it makes incidence onto thebeam splitter 6 as an s-polarized beam (that is, with its plane ofpolarization parallel to the surface of the beam splitter 6). Thus, apart of this incident beam is reflected by the beam splitter 6 accordingto the Brewster's law and only the portion of the light oscillating inthe perpendicular direction (or the p-polarized beam) propagates towardsthe target area of detection (as emitted light L1 of FIG. 3).

If this light is reflected by the reflector 200 (as shown in FIG. 3A),the reflected light L2 is elliptically polarized, including both lightoscillating in the perpendicular direction and light oscillating in thehorizontal direction. Most of the light oscillating in the horizontaldirection passes through the beam splitter 6 and is received by thelight-receiving element 43 after it passes through the ½ phase shifter41 and the second polarizer 42 with horizontal polarizer axis. In otherwords, a portion of the reflected light L2 is received by thelight-receiving element 43.

If the emitted light L1 is reflected by a target object 300 as shown inFIG. 3B, the reflected light L3 includes only components that oscillatein the perpendicular direction because only light oscillatingperpendicularly to the target object is made incidence to the targetobject. A portion of this reflected light is reflected again by the beamsplitter 6 according to the Brewster's law but the remaining portionpasses through the beam splitter 6. The portion that passes through thebeam splitter 6 also passes through the ½ phase shifter 41 but cannotpass through the second polarizer 42 and hence is not received by thelight-receiving element 43. In other words, reflected light L3 from thetarget object 300 is not received by the light-receiving element 43.Thus, the sensor can distinguish between reflected light from thereflector and reflected light from a target object. The beam splitter 6may be a half mirror without any polarization characteristic or apolarizing beam splitter with a polarization characteristic.

Let us consider next the situation where the direction of the polarizeraxis of the first polarizer 32 is horizontal and that of the secondpolarizer 32 is in the perpendicular direction. In this case, the lightemitted from the light-emitting element 31 and passed through the firstpolarizer 32 is linearly polarized, oscillating in the horizontaldirection. After this linearly polarized light passes through the ½phase shifter 33, it makes incidence onto the beam splitter 6 as thep-polarized beam, that is, it has the plane of polarizationperpendicular to the plane of the beam splitter 6. Thus, the incidentbeam passes through the beam splitter 6 according to the Brewster's lawand hardly any light is propagated into the target area of detection.This means that the structure shown in FIG. 4 is preferable in the caseof a retroreflective photoelectric sensor of the coaxial type. Thisshould be kept in mind also where the positions of the light-emittingand light-receiving elements 31 and 43 are interchanged.

Although FIG. 4 shows an embodiment wherein the ½ phase shifters 33 and41 are intimately in contact respectively with the first and secondpolarizers 32 and 42, a single ½ phase shifter may be used to replacethem, disposed between the beam splitter 6 and the common lens 5. FIG.14 shows a retroreflective photoelectric sensor 400′ thus structuredaccording to such an alternative embodiment of this invention having asingle ½ phase shifter 50 placed between the beam splitter 6 and thecommon lens 5. Its light-emitting and light-receiving optical systems 3′and 4′ are different from those of the sensor 400 shown in FIG. 4 inthat the phase shifters 33 and 41 are dispensed with. Thus, thisembodiment is advantageous in that the number of components is reducedand its production cost is accordingly lower.

As shown in FIGS. 12A and 12B, the retroreflective photoelectric sensor100 of the biaxial type embodying this invention may comprise a case101, a lens unit 105 forming the light-emitting lens 14 and thelight-receiving lens 21 in an integrated form together with atransparent cover 102, a linear polarizer sheet (as the first polarizer12) on the light-emitting side and another linear polarizer sheet (asthe second polarizer 13) on the light-receiving side. Symbols 13 and 22each indicate a ½ wave sheet (serving as a ½ wave shifter).

The lens unit 105 is a molded product of an acryl resin material withretardation value of less than 17 nm/mm, produced by an extrusionmolding process. The light-emitting lens 14 and the light-receiving lens21 are arranged on the back surface of the planar transparent cover 102where protrusions 114 and 115 are formed for engagement.

The case 101 is formed with conically shaped indentations 110 and 111respectively for providing a space for receiving the light-emitting lens14 and the light-receiving lens 21 of the lens unit 105.

The light-emitting element 11 serving as a light source, the linearpolarizer 12 and the ½ phase shifter 13 are mounted inside theindentation 110, and the light-receiving element 24, the linearpolarizer 23 and the ½ phase shifter 22 are mounted inside theindentation 111.

The lens unit 105 is attached to an attachment member in front of thecase 101 by engaging the protrusions 114 and 115 with indentations 116and 117 such that the light-emitting lens 14 and the light-receivinglens 21 are contained inside the indentations 110 and 111, respectively.The light-emitting element 11 is operated by a light-emitting circuit(not shown) and output signals from the light-receiving element 24 areinputted to a light-receiving circuit (not shown) such that the presenceor absence of an object is determined, depending on the quantity oflight received.

The quenching ratio of the lenses 14 and 21 is about 1/1000 and theirretardation value is less than 17 nm/mm. They are resin lenses producedby extrusion molding and cause only small deformations on the linearpolarization of the transmitting light. The polarizers 12 and 23 aredisposed respectively on the light-receiving side of the light-emittinglens 14 and the light-emitting side of the light-receiving lens 21 suchthat the light emitted from the light-emitting element 11 can be passedthrough the polarizer 12 to polarize it linearly and to be madeincidence onto the light-emitting lens 14 and the reflected light from areflector or a target object can be made incidence on thelight-receiving lens 21 and passed through the polarizer 23 to linearlypolarize it.

Since the lenses 14 and 21 are made of acryl resin and produced byextrusion molding, they can be formed integrally with the case 101 andhence a water-resistant and dust-resistant structure can be easilyformed and the number of individual components to be produced can bereduced. This also serves to provide a compact photoelectric sensor.

FIG. 13 is a sectional view of the photoelectric sensor 400 of thecoaxial type.

Symbols 401, 31, 32, 33, 41, 42, 43, 6 and 5 respectively indicate acase, the light-emitting element, the first polarizer, the first ½ phaseshifter, the second ½ phase shifter, the light-receiving element, thebeam splitter and the common lens.

With this sensor 400, too, the lens 5 is of an acryl material and isformed integrally with the case 401 and the optical system is of acoaxial type. Persons skilled in the art will easily understand theoperations of this sensor without further explanations.

In summary, the present invention provides retroreflective photoelectricsensors that can be produced inexpensively and are capable of detectinga light-reflecting target object reliably.

1. A retroreflective photoelectric sensor comprising: a light-emittingoptical system having a light-emitting element that emits an expandingbeam of light with a fixed gate angle, a first polarizer and alight-emitting lens arranged sequentially; a light-receiving opticalsystem having a light-receiving lens, a second polarizer and alight-receiving element arranged sequentially; a first phase shifterinserted between said first polarizer and said light-emitting lens; anda second phase shifter inserted between said second polarizer and saidlight-receiving lens, said first phase shifter and said second phaseshifter each serving to shift the phase by ⅜–⅝ with respect to thewavelength; wherein said first polarizer and said second polarizer havemutually perpendicular polarizer axes.
 2. The retroreflectivephotoelectric sensor of claim 1 wherein said light-emitting lens andsaid light-receiving lens are integrally formed by molding a plasticmaterial.
 3. A retroreflective photoelectric sensor comprising: alight-emitting optical system having a light-emitting element that emitsan expanding beam of light with a fixed gate angle, a first polarizerand a light-emitting lens arranged sequentially; a light-receivingoptical system having a light-receiving lens, a second polarizer and alight-receiving element arranged sequentially, said first polarizer andsaid second polarizer having mutually perpendicular polarizer axes; andmeans disposed between said first polarizer and said light-emitting lensfor serving to shift the phase by ⅜–⅝ with respect to the wavelength andcanceling the total rotation of the polarization plane that is the sumof rotations caused by passing through said first polarizer and saidlight-emitting lens.